172 lines
7.1 KiB
HTML
172 lines
7.1 KiB
HTML
<html>
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<head>
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<link href="style.css" rel="stylesheet" type="text/css" />
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<title>osmocom-analog</title>
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</head>
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<body>
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<center><table><tr><td>
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<h2><center>Radio setup</center></h2>
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<center><img src="setup.jpg"/></center>
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<p>
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The main idea of this project is to use a sound adapter to generate and analyze FM modulated signals.
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Only a transmitter and a receiver is required.
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All signal processing is done in software.
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The software is written for Linux.
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So the first thing you need is PC with a sound adapter.
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This software uses <a href="http://www.alsa-project.org">Alsa</a> kernel driver to access audio input and output.
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</p>
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<center><img src="pc+sound.jpg"/></center>
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<p>
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Use a radio transmitter for 2-meter band to transmit to A-Netz or B-Netz phones.
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Use a radio transmitter for 70-cm band to transmit to C-Netz or NMT phones.
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An amateur radio can be used, but be sure it is not limited to amateur radio frequencies and channel spacing.
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For A-Netz the transmitter must be able to transmit around 162 MHz.
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For B-Netz the transmitter must be able to transmit around 153 MHz.
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For C-Netz the transmitter must be able to transmit around 465 MHz.
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For NMT the transmitter must be able to transmit around 463 MHz.
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</p>
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<p>
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This image shows a two-band amateur radio transceiver without limitations for the frequencies above:
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</p>
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<center><img src="transmitter.jpg"/></center>
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<p>
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I built several transmitters out of modified taxi radios.
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It features a simple digital PLL (configurable by jumpers) and allows to directly connect the audio signal to the oscillator.
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This is highly recommended for the C-Netz, because signaling is done by modulating (shifting) the carrier directly.
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One unit has a switch to change between A-Netz and B-Netz (2-meter version) and the other units to change between C-Netz and NMT (70-cm version).
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I added an input to switch between two channels for the B-Netz.
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(Whenever a B-Netz phone is paged, the channel must be temporarily switched to the paging channel.)
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</p>
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<center><img src="transmitter2.jpg"/></center>
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<p>
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Use radio receiver for 2-meter band to receive from A-Netz or B-Netz phones.
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Use radio receiver for 70-cm band to receive from C-Netz or NMT phones.
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A trasceiver cannot be re-used, because all networks are full duplex.
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A separate receiver is required to simultaneously receive while the transmitter is transmitting.
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For A-Netz the receiver must be able to receive around 162 MHz and 157 MHz.
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For B-Netz the receiver must be able to receive around 153 MHz and 148 MHz.
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For C-Netz the receiver must be able to receive around 465 MHz and 455 MHz.
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For NMT the receiver must be able to receive around 463 MHz and 453 MHz.
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The receiver must support both up-link and down-link frequencies, to make a test loop for the calibration process.
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</p>
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<p>
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This image shows a two-band amateur radio receiver that supports all the frequencies:
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(It is not sufficient for C-Netz, because it distorts the signal too much.)
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</p>
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<center><img src="receiver.jpg"/></center>
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<p>
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The best solution I could find for low price is the Uniden UBC360CLT.
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It is actually some kind of clock radio with scanner.
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It supports all relevant bands up to about 950 MHz.
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In all bands the channel spacing is freely selectable.
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Up to 300 channels can be stored and simply selected by using up/down keys.
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</p>
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<center><img src="receiver2.jpg"/></center>
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<p>
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I added a discriminator output to the radio, to get undistorted receive signal.
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This is highly recommended for the C-Netz, because signaling is done by sifting the carrier directly.
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</p>
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<center><img src="discriminator.jpg"/></center>
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<p>
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In order to keep RF emission low, use a dummy load for lab test.
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Connect one dummy load to the transmitter and another one to the mobile phone's antenna connector.
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Alternatively use two separated antennas for outdoor use of the receiver and transmitter, to avoid using a duplexer unit.
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Also use antanna on the mobile phone to move further away from the base station.
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</p>
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<center><img src="dummyload.jpg"/></center>
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<p>
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For the radios and the phone, you nee a sufficient power supply.
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Especially for old A-Netz phones, be sure to have something strong with 10 Ampere or even more, depending on your phone.
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My phone did not work correctly with a 5 Ampere 12 Volts regulated power supply.
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I suggest to use an amateur radio power supply with low ripple.
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In my case I use modified ATX-2 power supply for 12 or 13.8 Volts.
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The voltage can be adjusted in most power supplies, but 12 Volts is good enough.
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These power supplies have built-in over-current/over-voltage protection.
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The ATX-2 power supplies are regulated on the 12 Volts rail.
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I added two plugs and a switch.
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The switch connects to the PS-On line.
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In case of an over-current, the switch can be used to reset the protection circuit.
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<font color="red"><b>
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Note that opening a switching power supply can be dangerous and may cause death.
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If you don't have the knowledge about power supplies, ask an expert to help you.
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</b></font>
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</p>
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<center><img src="power.jpg"/></center>
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<p>
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Early tests were done without any wiring between radios and sound adapter.
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This may worked for early tests with the B-Netz, but turned out to be a pain in the ass.
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It is not possible to keep the levels and avoid feed-backs.
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</p>
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<center><img src="headphone.jpg"/><img src="microphone.jpg"/></center>
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<p>
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The sound adapter must be connected to a transmitter and a receiver.
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Coils are suggested for galvanic separation.
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I don't use coils, instead I add extra grounding wire between earth (power supply) and sound interface.
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For adjustmnt I highly recommend to use variable resistors.
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This makes the adjustment process faster.
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I use 4.7 K resistor for connecting headphone output to radio input.
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I use 10 K resistor for connecting radio output (discriminator) to microphone input.
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</p>
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<center><img src="poti.gif"/></center>
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<p>
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Connect the ground of input and output to one end of the resistor. (shown in the picture as 'Masse')
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Select this side to which the slider moves, if the knob turned all the way left (or moved all the way down).
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Connect the input signal ('Signaleingang') to the other end of the resistor.
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Finally connect the output signal ('Signalausgang') to the slider of the resistor.
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</p>
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<p>
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During development process, I ended up like this:
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</p>
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<center><img src="chaos.jpg"/></center>
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<p>
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After figuring out how everything works, I decided to build a small box with knobs on the front and 3.5mm audio jacks in the back.
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It allows to connect two radio transmitters and two radio receives to one stereo sound card.
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Also it has a banana plug to ground the box.
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</p>
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<center><img src="stellwerk.jpg"/></center>
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<p>
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And finally you need a classic working phone.
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Be sure to connect the phone to a dummy load too.
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Place both dummy loads (transmitter and phone) close together (several centimeters).
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Put an antenna on the receiver.
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If your phone has built in antenna and cannot be connected to a dummy load (C-Netz / NMT), stay a few meters away from transmitter (dummy load) and receiver.
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</p>
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<center><img src="b-netz-display.jpg"/></center>
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<hr><center>[<a href="index.html">Back to main page</a>]</center><hr>
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</td></tr></table></center>
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</body>
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</html>
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